Add guide for DNS-Over-HTTPS (DoH) using dnscrypt-proxy

[faf0]

What does this PR aim to accomplish?:

Add a guide for installing dnscrypt-proxy for DNS-Over-HTTPS (DoH).

How does this PR accomplish the above?:

Adds a new guide under Guides -> DNS.

Link documentation PRs if any are needed to support this PR:

This guide is based on https://ffoerg.de/posts/2024-01-28.shtml by the same author.

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